1. Field of the Invention
The present invention relates to a temperature compensation circuit for a Wheatstone bridge sensor, and more particularly to voltage and current regulators to compensate for variations in the load current resulting from temperature changes.
2. Description of the Prior Art
The operating parameters for elements of a pressure sensor, including the Wheatstone bridge, can vary nonlinearly with a change in temperature. These fluctuations in the operating parameters may cause inaccurate sensor readings. Various methods have been employed by the prior art to compensate for sensor inaccuracies arising from such environmental factors in a variety of sensors including the Wheatstone bridge.
U.S. Pat. No. 4,096,748 issued to Michel Pichon on Jun. 27, 1978, describes a Wheatstone bridge for measuring temperatures by measuring changes in the impedance in the bridge caused by different temperatures. Pichon does not attempt to compensate for any operating parameter of the bridge.
U.S. Pat. No. 5,069,066 issued to Robert S. Djorup on Dec. 3, 1991, describes a constant element anemometer having two Wheatstone bridges. A four-arm Wheatstone bridge is operated as a single arm of a second four-arm Wheatstone bridge. A feedback controlled constant resistance operation is achieved in the bridge within a bridge configuration.
U.S. Pat. No. 5,076,376 issued to Bruno Bizet et al. on Dec. 31, 1991; and GB Patent Specification No. 1,263,182 for Geoffrey Cyril Cooke published on Feb. 9, 1970, both describe strain gauge weighing devices. Neither attempts to compensate for an operating parameter of the bridge.
U.S. Pat. No. 5,163,325 issued to Craig W. White et al. on Nov. 17, 1992, describes self-compensating accelerometer having a leg of resistors connected to the strain gage and controlled by a microprocessor to compensate for temperature variations and manufacturing tolerances. The resistance is varied when the microprocessor switches fixed resistors in parallel connection with the leg of the bridge.
U.S. Pat. No. 5,163,326 issued to Roger L. Frick on Nov. 17, 1992, describes a circuit for compensating a pressure sensor for changes in static pressure, which can cause stray capacitances and non-linearities in the output from a capacitive pressure sensor. The compensation circuit has a capacitive value fixed relative to the capacitive pressure sensor. Frick compensates for static pressure, not variations in temperature.
Japanese Kokai No. 1-145574 filed by Fujikura, Ltd. for Tatsuya Ito and published on Jun. 7, 1989, describes an acceleration sensor including a wheatstone bridge which is balanced by varying the resistance on opposite legs of the bridge. Correction resistances are interposed in series between reference resistances, and correction resistances are selected by connecting a terminal at a node of the reference resistance and the correction resistance to an output terminal.
Though these and other patents disclose circuitry for Wheatstone bridge sensors, the known prior art does not disclose or suggest the use of the constant current regulator of the present invention to minimize the effect of quiescent current variation with temperature. None of the above patent references, either alone or in combination with one another, is seen to describe the instant invention.